An active matrix substrate used for a liquid crystal display device and the like is provided with a switching element, e.g., a thin film transistor (hereafter referred to as a “TFT”), on a pixel basis. To date, a TFT including an amorphous silicon film as an active layer (hereafter referred to as an “amorphous silicon TFT”) and a TFT including a polycrystalline silicon film as an active layer (hereafter referred to as a “polycrystalline silicon TFT”) have been widely used for such a switching element.
In recent years, it has been proposed to use an oxide semiconductor, instead of an amorphous silicon or a polycrystalline silicon semiconductor, as the material for forming the active layer of the TFT. Such a TFT is called an “oxide semiconductor TFT”. An oxide semiconductor has higher mobility than amorphous silicon. Consequently, the oxide semiconductor TFT can operate at a higher speed than the amorphous silicon TFT. In addition, the oxide semiconductor film is formed by a simpler process than the process for a polycrystalline silicon film and, therefore, can be applied to a device that is required to have a large area.
Regarding the semiconductor device provided with the oxide semiconductor TFT, if a layer, e.g., a copper (Cu) layer or an aluminum (Al) layer, having low electrical resistance is used for a source and drain electrode, the contact resistance between the Cu layer or Al layer and the oxide semiconductor layer serving as the active layer may increase, and there is a likelihood that predetermined TFT characteristics are not obtained. In order to address this problem, formation of a Ti film between the Al layer or Cu layer and the oxide semiconductor layer has been disclosed (for example, PTLs 1 and 2).
Meanwhile, in a TFT having a bottom gate structure, when a Cu layer or an Al layer is used for a gate electrode, a metal layer, e.g., a Ti layer, may be formed between the Cu layer or the Al layer and a glass substrate so as to enhance the adhesiveness between the two (for example, PTL 2).